grant to Ernst Back of a ScandinavianAmerican Fellomhip. TABLE
a
=
A.
=
A,
=
OF SYMBOLS
absorptivity of diffusing material absorbance of total diffusing material a t the boundarv
(:)
log,, = point absorbance a t oosition x = loglo = observed total absorbance when slit midpoint is a t position x = concentration of diffusing niaterial a t any particular position x = concentration of diffusing material at the boundary where
(9)
A, C Co
x = o =
factor
=
2.303 AoP,
factor
=
[I
+$I
f1
=
9
= -djQ/dzE constant io
s and x
h
i and
+s
X
within x
-
= height of the light beam i = intensitv of transmitted
light a t position x and a t gny other position x , respectively io and j o = intensity of incident light a t position x and a t any other position x , respectively I = total transmitted radiation energy Io = total incident radiation energy 1 = path of light beam through a solution. ?in. Po,P I = coefficients of assumed linear concentration ratio-distance equation = constant = -2.303 alCo = half width of slit S
4A,
distance from boundary to cell position a t the slit center = slit width correction
=
LITERATURE CITED
(1) Back, E., Felicetta, V. F., XcCarthy, J. L., AXAL.CHEJI.29, 1903 (1957). (2) Beckman Instruments, Inc., Fullerton, Calif., Instruction Manual 305-A. (3) Dwight, H. B., “Tables of Integrals and Other Mathematical Data,” p. 126, Maemillan, Sen, York, 1950. (4) Felicetta, V. F., Bhola, A., McCarthy, J. L., J . Ana. Chem. SOC. 78, 1899 (1956). \ Felicetta. T’. F.. Markham, A. E.,
RECEIVED for revien Septeml,er 11, 1957. -4ccepted June 9, 1958.
Determination of the Metal Content of Paint Driers EDTA Titration in Alcohol-Benzene Solution CLAUDE A. LUCCHESI and CLYDE F. HlRN Analytical Research Deparfment, The Sherwin- Williams Co., Chicago, 111.
F A simple titrimetric method for the determination of the metal content of calcium, cobalt, lead, manganese, and zinc driers i s rapid, precise, and yields results in close agreement with those obtained b y gravimetry and flame photometry. The new method i s based upon the reaction of metal ions w:th the chelating agent, (ethylenedinitril0)tetraacetic acid, EDTA. Previously reported chemical methods necessitated the decomposition of the drier and the isolation of the metal ion before the metal content could be determined. In the method described decomposition i s eliminated. The drier i s dissolved in an alcohol-benzene solution, a known excess of standard EDTA is added, and the excess i s back-titrated with standard zinc chloride to the Eriochrome Black T end point. Only 0.2 gram of sample is required, and a single determination can be made in 10 minutes. The coefficent of variation of the method i s estimated to b e 0.8%.
I
industrj- catalysts n-hich accelerate the drying of paint and varnish films are known as driers. The driers most commonly used are metallic salts of organic acids or some type of metal-organic acid complex (8, 9). The metal content of the driers is the most critical factor in determining their cost and performance. Consequently, the control of the concentraN THE PAIST
tion of the metal n-ithin rather narrow liniits is a problem in the manufacture and use of driers. This research makes possible the rapid determination of calcium, cobalt, lead, manganese, or zinc in a given drier by a single, simple method. Man)- satisfactory chemical methods for the determination of the ions of these metals in aqueous solution are known (IO). I n applying these methods t o drier analysis, the drier must first be decomposed and the metal ion isolated. Once isolated, each metal ion generally must be determined by a different method. For example. the American Society for Testing Materials ( I ) specifies different procedures for the determination of the ions of cobalt, lead, manganese, and zinc. The cobalt is precipitated as the nitroso-2-naphthol complex, lead is precipitated as the sulfate, manganese is oxidized t o permanganate which is then titrated with standard vanadyl sulfate, and zinc is titrated with standard potassium ferrocyanide. Recently, flame photometric methods for determining the metal content of calcium, cobalt. lead, and manganese driers have been reported ( 2 , 7’). The flame method \?-as a tremendous improvement over existing methods and, with the exception of zinc, made possible the direct determination of the metal content of all the common driers n-ithout first decomposing the drier. The disadvantages of this method are
the cost and maintenance of flame equipment and the necessity for preparing and storing calibration standards. The method reported here requires only coninion laboratory glassware and is based upon the well known reaction of metal ions with the chelating agent, the disodium salt of (ethylenedinitri1o)tetraacetic acid (EDTA) (12). TKOchelometric methods have been reported for the determination of drier metals n-hich involve an acid hydrolysis step for isolating the metal ions. I n one ( l l ) ,the drier is dissolved in chloroform and the metal ion is extracted rvith 1 t o 1 hydrochloric or nitric acid. I n the other procedure ( 6 ) , the acid hydrolysis is carried out by heating the drier t o boiling with 1Oyohydrochloric acid. The aqueous solution of the metal ion is then titrated directly or indirectly t o the Eriochrome Black T or murexide indicator end point. The method presented does not involve a decomposition step. The drier is dissolved in an organic solvent, a knon n excess of standard EDTA solution is added, and the excess EDTA is back-titrated n i t h a standard zinc solution to the Eriochrome Black T end point. This procedure works for all the driers, including zinc. Its simplicity and speed combined lvith nominal cost make it ideally suited for the production control analysis of naphthenate and octoate driers. During the past several months in n-hich the VOL. 30,
NO. 11, NOVEMBER 1958
1877
Table I.
Comparison of Drier Methods
EDTA Methods AhGraviAcid holmetric Flame extract benzene Cobalt in Cobalt Naphthenate 6.15 6.10 6.15 6.09 6.13 6.15 6.16 6.07 6.11 6.03 6.14 6.11 6.10 6.05 6.16 6.04 6.11 6.03 6.13 6.16 Av. 6.12 6.07 6.15 6.09 Std. dev. 0.020 0.052 0.013 0.046 yo Lead in Lead Naphthenate 25.11 25.3 25.07 25.02 25.14 25.6 25.18 24.95 25.03 2 1 . 8 25.37 24.87 25.11 25.6 25.30 25.01 25.18 25.6 25.04 25.40 Av. 25.11 25.4 25.19 25.05 Std. dev. 0.055 0.36 0.143 0.205 % ’ Calcium in Calcium Naphthenate 4.13 4.17 4.11 4.09 4.11 4.20 4.11 4.13 4.06 4.09 4.17 Av. 4.10 4.19 4.10 4.13 % Manganese in Manganese Naphthenate 5.87 5.67 5.73 5.73 5.85 5.63 5.70 5.67 5.82 5.72 5.73 dv. 5.85 5.65 5.72 5.71 70Zinc in Zinc Naphthenate 7.82~ ... 8.09 7.98 7.90 ... 8.05 7 . 9 4 7.91 ... 8.05 8.04 Av. 7.88 ... 8.06 7.99 a Ferrocyanide method. method has been used to control factory operations, it has given completely satisfactory results. A single determination can be made in 10 minutes. REAGENTS A N D APPARATUS
0.01M EDTA solution. Weigh 3.7225 grams of disodium (ethylenedinitri1o)tetraacetate, dissolve in distilled water, and dilute to 1 liter. Store in a polyethylene or borosilicate glass bottle. EDTA, disodium salt, purchased from Eastman Organic Chemicals was satisfactory. 0.01M zinc solution. Weigh about 0.6538 gram of reagent grade zinc to the neaEkst 0.1 mg.-and &ssolve in 25 ml. of 1 to 3 hydrochloric acid. Warm if necessary. Dilute to 1.00 liter in a volumetric flask. Reagent grade zinc from J. T. Baker Chemical Co. was satisfactory. Calculate the exact molarity of the solution with the expression. (weight of zinc) (0.01) Molarity = 0.6538 Buffer solution. Add 350 ml. of concentrated ammonium hydroxide to 54 grams of C.P. ammonium chloride and dilute t o 1 liter with distilled water. Indicator mixture. Triturate 0.2 gram of Eriochrome Black T and 100
1878
ANALYTICAL CHEMISTRY
grams of ACS reagent grade sodium chloride, and store the mixture in a tightly stoppered bottle. This mixture is apparently stable for years. Eriochrome Black T purchased from Eastman Organic Chemicals was satisfactory. Chemicals. Tartaric acid, c.P., ascorbic acid. c.P.. benzene., c.P.., and alcohol, 95%. ‘ Weighing assembly. An eyedropper fitted into a cork which in turn fits into a 50-ml. Erlenmeyer flask is a convenient way to handle the drier for weighing. Magnetic stirrer. A magnetic stirrer and a Teflon-coated stirring bar were used to ensure thorough mixing during the titration. This device also makes possible a more critical observation of the end point. Magne-Stir (Arthur S. La Pine Bt Co.) was used. PROCEDURE
Standardization of EDTA Solution. Measure 40.00 ml. of the EDTA solution into a 400-ml. beaker which contains 2 ml. of benzene and 50 ml. of alcohol. Add 10 ml. of buffer solution and 0.2 gram of indicator mixture. Titrate with the standard zinc solution t o the first appearance of a red color. Calculate exact molarity of the EDTA solution with the expression Molarity of EDTA = (molarity of zinc soln.) (ml. zinc s o h ) 40.00 Calcium, Cobalt, and Zinc. Weigh 0.2 gram of the drier to the nearest 0.1 mg. into a 250-ml. Erlenmeyer flask. (Ten drops of drier equal about 0.2 gram.) Add 2 ml. of benzene and mix. Then add 50 ml. of alcohol and mix again. Place 40.00 ml. of EDTA in the flask, and add 10 to 15 ml. of buffer solution. A cloudiness may develop a t this point or later, but this does not affect the end point. Add about 0.2 gram of indicator mixture and mix thoroughly. Keeping the solution well mixed, titrate with the standard zinc solution to the first appearance of red as seen in the standardization procedure. Calculate the metal concentra tio n . Lead. Follow the above procedure and, in addition, add a spatula tip of tartaric acid before adding the buffer solution. Manganese. Follow the above procedure, but instead of the tartaric acid, add about 0.1 gram of ascorbic acid before adding the buffer solution. Calculations. The percentage of metal in the drier may be calculated with the expression % metal = [(ml. EDTA soln.) (molarity EDTA) (ml. Zn soln.) (molarity Zn)] ( A ) (100) weight of sample in mg.
where A is 40.08 for calcium, 58.94 for cobalt, 65.38 for zinc, 207.21 for lead, and 54.94 for manganese. RESULTS
Typical results obtained with the
recommended procedure are given in Table I. The gravimetric determination of calcium in the calcium drier was made by first decomposing the drier with hot concentrated sulfuric acid. The sample was then ignited and the calcium mas weighed as calcium sulfate. Cobalt was determined by extracting the cobalt ion with hot dilute hydrochloric acid, adjusting the pH to about 8 with ammonium hydroxide, and precipitating with diammonium acid phosphate. The precipitate was ignited and weighed as the pyrophosphate. Lead was separated from the drier, simultaneously precipitated as the sulfate by treatment with dilute sulfuric acid, and weighed as lead sulfate. The manganese was determined in essentially the same way as the cobalt. Zinc was extracted with dilute hydrochloric acid and titrated with potassium ferrocyanide to an external uranium acetate end point. The flame photometric results were obtained by a procedure similar to the one suggested by Cox (2). The two sets of EDTA results were obtained in exactly the same way except that the set labeled Acid Extract was obtained in aqueous solution. The metal ion was brought into aqueous solution by decomposing the drier by acid hydrolysis. After the hydrolysis, the free organic acid procedure was removed by filtration, and the metal ion in the filtrate was determined with EDTA. The same acid hydrolysis procedure was used by Leggieri (6). DISCUSSION
The recommended procedure is probably not the best one for each metal, but it makes possible the determination of all the drier metals under essentially the same set of conditions. It might have been possible t o devise procedures for the direct titration of the metal ions. It was felt, however, that the simplicity achieved by a single procedure far outweighed the possible gain in precision anticipated by tailor-made procedures for each drier. The relatively small sample sizes and the 0.01M reagents were originally selected because of the anticipated interference of the pink color of the cobalt drier (5) and also because the particular sample size used (about 10 drops) n-as convenient to handle. A standard zinc chloride solution was used as the backtitrant because it can be prepared from a primary standard. Cobalt. The only determination which is a t all critical is that involving cobalt. Cobalt ion must be backtitrated because it forms so strong a complex with the indicator that E D T A removes the cobalt slowly and a very poor end point is obtained (IS). Even in the back-titration, the
cobalt-indicator complex is so strong that it can cause errors. Apparently, as the end point of the back-titration is neared and the concentration of the excess EDTA becomes very small compared to that of the indicator, the cobalt tends t o leave the EDTA complex and to form a complex with the indicator. This may lead to a premature end point and high results. By back-titrating rapidly, however, satisfactory results can be obtained (3, I S ) . To determine the effect of the total time of the backtitration on the accuracy of the method, nine samples of a cobalt drier were titrated for varying lengths of time and a t somewhat different rates. Results are given in Table 11. Samples 1, 2, 3, 4, 5, 6, and 8 were back-titrated a t a fairly constant rate from start to end point. Sample 7 was back-titrated by first adding 80% of the titrant as quickly as it would leave the buret, and then by adding the remainder dropwise until the end point was reached. Sample 9 was titrated by adding all of the bark-titrant dropwise. Data in Table I1 show that the average of the results of the first seven determinations is 6.07%. The average of earlier results which were obtained before the time dependency was realized is 6.09% (Table I). Apparently, as long as the back-titration is completed within about 21/* minutes, the results agree well with those obtained by other methods. Solvent. The work of Gerhardt and Hartmann (4) illustrated that EDTA titrations can be carried out in a solvent which is largely nonaqueous. Acetone, the solvent used by Gerhardt and Hartmann for lubricating oils, was not satisfactory for driers because most driers are insoluble in acetone. Ethyl alcohol was tried and found to be satisfactory for all but the lead driers. Occasionally, a manganese drier would not dissolve in the alcohol and dioxane was used for lead and manganese driers. The mixtures had t o be heated to effect
solution in alcohol or dioxane. The best combination of solvents for all the driers proved to be benzene and alcohol. By adding 2 ml. of benzene and then 50 ml. of alcohol, the heating step could be eliminated. The benzene apparently holds the drier in a true solution which can then be diluted with alcohol. The disadvantage of this combination is that the benzene is responsible for the cloudiness which develops after the aqueous titrant is added to the sample solution. The cloudiness apparently makes the end point a little more difficult to detect. However, the results obtained in the alcohol-benzene solution agree well with those obtained in alcohol alone or in dioxane. Precision and Accuracy. On the basis of the cobalt and lead results, the coefficierft of variation of the method is estimated to be 0.8%. This figure is comparable with that obtained by Gerhardt and Hartmann (4) for the titration of zinc additives in lubricating oils. The precision of the EDTA titration in alcohol-benzene, as measured by the standard deviation, is significantly poorer than the same titration in aqueous media. The poorer precision is attributed to the fact that the end point in the alcohol-benzene titration must be detected in the presence of a turbidity. It is difficult to determine the accuracy of the method because of the lack of standards. The results obtained by various methods are shown in Table I. If it is presumed that the gravimetric method gives the true metal content of the driers, then a comparison of the averages of the EDTA alcoholbenzene method Kith those of the gravimetric method leads to the relative accuracies: calcium, +0.73%; cobalt, -0.49%; lead, -0.24%; manganese, -2.4%; and zinc, +1.4%. ACKNOWLEDGMENT
The authors wish t o acknowledge
Table II. Effect of Total Time of BackTitration on Cobalt Results
Sample KO.
Time, See. 30
1 2
Cobalt,
%
6.07 5.96 6.04 6.09
40 60 80
3 4
the assistance of N. K. Kaprielyan who obtained most of the flame photometric data. The helpful suggestions of J. C. Weaver and the encouragement of R. F. Schneider, K. R. Brown, and Richard Tobin are also gratefully acknowledged. LITERATURE CITED
(1) Am. SOC.Testing Materials, Designa-
tion D 56447, “Standard Methods of Testing Liquid Driers,’’ 1947. (2) Cox, D. S., Paint and Varnish Production 45 (1955). ( 3 ) Flaschka, A., Barnard, A. J., Jr., Broad, W. C., Chemist Analyst 47, 25 (1958). (4)Gerhardt, P. B., Hartmann, E. R., ANAL.CHEW29, 1223 (1957). (5) Harris, W. F., Sweet, T. R., Ibid., 26, 1649 (1954). (6) Leggieri, G., Chimica ( M i l a n ) 10, 287-8 (1955). (7) Lucchesi, C. A , , Ofic. Dig.Federation
Paint & Varnish Production Clubs 30,
212-30 (1958). (8) Mattiello, J. J., “Protective and Decorative Coatings,” Vol. I, p. 499, Wiley, Kern York, 1942. (9) Pap:: H. F., “Organic Coating Technology, Vol. I, p. 227, Kiley, New York, 1954. (10) Ibid., p. 240. (11) Pokorny, J., Pribyl, J., Chem. zvesti. 9, 20-6 (1955). (12) Welcher, F,. J., “Analytical Uses of Ethylenediamlne Tetraacetic hcid,” Van Sostrand, Ken- York, 1958. (13) Ibid., p. 231. RECEIVEDfor review May 8, 1958. Accepted July 29, 1958.
Simple Microspectrophotometer DONALD
F. tl.
WALLACH and DOUGLAS
M. SURGENOR
Department of Biological Chemistry, Harvard Medical ‘School, and Protein Foundation, Boston, Mass.
b A spectrophotometer is described which permits rapid and accurate absorption measurements on 10 PI. of solution. A light beam, 0.4 mm. in diameter, is obtained from a glass prism monochromator. The cuvette is fabricated from capillary tubing, 1 mm. in inside diameter. It is positioned with the aid of a microscope, after which the transmitted light is
deflected to a photomultiplier. The ratio l o / / is measured with a balanced photomultiplier bridge circuit.
an investigation into the chemistry of human blood platelets and other tissue cells, the need arose for rapid and simple spectrophotometric measurement of several constituents, particularly nitrogen, calcium, and URIXG
phosphorus, in very limited amounts of tissue. Although there is considerable literature on the spectrophotometry of cells and tissues ( 5 ) , techniques for simple microspectrophotometry are relatively few ( I ) , and, when applied to truly small volumes, difficult and costly. A simple microspectrophotometer on the general principle of the microcolorirneter of Holter and Lgvtrup VOL. 30, NO. 1 1 , NOVEMBER 1958
0
1879
